1. Field of the Invention
The present invention relates to a method for transmitting an uplink signal in a multiple antenna scheme, and more particularly, to a method for acquiring precoding information efficiently and transmitting an uplink signal using the precoding information in a multiple antenna scheme.
2. Discussion of the Related Art
The present invention relates to a method for transmitting MIMO is short for Multiple Input Multiple Output. Beyond conventional schemes using a single Transmit (Tx) antenna and a single Receive (Rx) antenna, MIMO uses a plurality of Tx antennas and a plurality of Rx antennas to thereby increase the transmission and reception efficiency of data. With the use of multiple antennas at a transmitter or a receiver, MIMO seeks to increase capacity or improve performance in a wireless communication system. The term “MIMO” is interchangeable with “multiple antenna”.
The MIMO technology does not depend on a single antenna path to receive an entire message. Rather, it completes the message by combining data fragments received through a plurality of antennas. Because MIMO may increase data rate within a certain area or extend system coverage at a given data rate, it is considered as a promising future-generation mobile communication technology that may find its use in a wide range including mobile terminals, relays, etc. With the growth of data communication, MIMO is attracting attention as a future-generation technology that may overcome a limit on transmission capacity that is almost reached due to the increased data communication.
FIG. 1 illustrates the configuration of a typical MIMO communication system. Referring to FIG. 1, a simultaneous increase in Tx antennas of a transmitter to NT and in Rx antennas of a receiver to NR increases a theoretical transmission capacity in proportion to the number of antennas, compared to use of a plurality of antennas at only one of the transmitter and the receiver. Therefore, transmission rate is increased and frequency efficiency is remarkably increased. Given a maximum transmission rate Ro that may be achieved in case of a single antenna, the increase of channel capacity may increase the transmission rate, in theory, to the product of Ro and Ri in case of multiple antennas. Ri is a transmission rate increase rate.
For instance, a MIMO communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna system. Since the theoretical capacity increase of the MIMO system was proved in the middle 1990's, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
Concerning the research trend of MIMO, active studies are underway in many respects of MIMO, inclusive of studies of information theory related to calculation of multi-antenna communication capacity in diverse channel environments and multiple access environments, studies of measuring MIMO radio channels and MIMO modeling, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc.
There are two types of MIMO schemes: spatial diversity and spatial multiplexing. Spatial diversity increases transmission reliability using symbols that have passed in multiple channel paths, whereas spatial multiplexing increases transmission rate by transmitting a plurality of data symbols simultaneously through a plurality of Tx antennas. Taking advantages of these two schemes by using them in an appropriate combination is a recent active study area.
To describe a communication scheme in a MIMO system in detail, the following mathematical model may be used.
It is assumed that there are NT Tx antennas and NR Rx antennas as illustrated in FIG. 1. Regarding a transmission signal, up to NT pieces of information can be transmitted through the NT Tx antennas, as expressed as the following vector.s=[s1,s2, . . . ,sNT]T  Equation 1
A different transmit power may be applied to each piece of transmission information s1, s2, . . . , sNT. Let the transmit power levels of the transmission information be denoted by P1, P2, . . . , PnT, respectively. Then the power-controlled transmission information ŝ may be given as [Equation 2].ŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2,PNTsNT]T  [Equation 2]
ŝ may be expressed as a diagonal matrix P of transmit power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                                                0                      ⁢                                                                                                                                                                                                                                                                                                                                                                          P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          3                ]            
Meanwhile, actual NT transmitted signals x1, x2, . . . , xNT may be configured by applying a weight matrix W to the power-controlled information vector ŝ. The weight matrix W functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel statuses, etc. These transmitted signals x1, x2, . . . , xNT are represented as a vector x, which may be determined as
                                                        x              =                            ⁢                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        i                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                                                                                        =                            ⁢                                                [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                w                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              w                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    j                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            T                                                                                                                                ]                                                                                                        =                            ⁢                              W                ⁢                                  s                  ^                                                                                                        =                            ⁢              WPs                                                          [                  Equation          ⁢                                          ⁢          4                ]            
wij denotes a weight for a jth piece of information ŝj transmitted through an ith Tx antenna and the weights are expressed as the matrix W. W is referred to as a weight matrix or a precoding matrix.
The afore-mentioned transmitted signal x may be considered in two cases: spatial diversity and spatial multiplexing.
In spatial multiplexing, different signals are multiplexed prior to transmission. Accordingly, the elements of the information vector s have different values. In contrast, the same signal is transmitted in a plurality of channel paths in spatial diversity. As a result, the elements of the information vector s have the same value.
Spatial multiplexing and spatial diversity may be used in combination. For example, the same signal may be transmitted through three Tx antennas in spatial diversity, while different signals may be transmitted through the other Tx antennas in spatial multiplexing.
Given NR Rx antennas, signals received at the Rx antennas, y1, y2, . . . , yNR may be represented as the following vector.y=[y1,y2, . . . ,yNR]T  [Equation 5]
When channels are modeled in the MIMO communication system, they may be distinguished according to the indexes of Tx and Rx antennas and the channel between a jth Tx antenna and an ith Rx antenna may be represented as hij. It is to be noted herein that the index of the Rx antenna precedes that of the Tx antenna in hij.
The channels may be represented as vectors and a matrix by grouping them. The vector representation of channels may be carried out in the following manner.
FIG. 2 illustrates channels from NT Tx antennas to an ith Rx antenna.
Referring to FIG. 2, the channels from the NT Tx antennas to the ith Rx antenna may be expressed as [Equation 6].hiT=[hi1,hi2, . . . ,hiNT]  [Equation 6]
Also, all channels from NT Tx antennas to NR Rx antennas may be expressed as the following matrix.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          7                ]            
Actual channels experience the above channel matrix H and then are added with Additive White Gaussian Noise (AWGN). The AWGN n1, n2, . . . , nNR added to the NR Rx antennas is given as the following vector.n=[n1,n2, . . . ,nNR]T  [Equation 8]
From the above modeled equations, the received signal is given as
                                                        y              =                            ⁢                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                  N                          R                                                                                                                    ]                                                                                        =                            ⁢                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                    …                                                                                                      h                                                          1                              ⁢                                                              N                                T                                                                                                                                                                                                                                      h                            21                                                                                                                                h                            22                                                                                                    …                                                                                                      h                                                          2                              ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                                                                                        =                            ⁢                              Hx                +                n                                                                        [                  Equation          ⁢                                          ⁢          9                ]            
In a 3GPP LTE system attracting attention as a future-generation mobile communication technology, the above-described MIMO operation applies only to downlink signal transmission from a Base Station (BS) to a User Equipment (UE). As efforts are continuous to increase transmission rate and achieve diversity gain also from uplink signal transmission, there exists a need for specifying this uplink MIMO technology in upcoming standards.